Peavey CS-800 -- Keeps Blowing Fuses

Hi, I've got an old 4U Peavey CS-800 that I bought as-is. The previous owner claimed it worked when I bought it.

It keeps blowing fuses, almost instantly on powerup. This does not happen if only Channel A's power is hooked up, and it does happen if only Channel B is hooked up, or if both channels are. I also swapped the channel boards and the "B" board is the only one that causes failure.

I tested all the transistors on Channel B and found one to be bad, so I replaced it -- and the fuse blowing still occurs.

OK, so power amp B is blown, but powr amp A by itself will work, right?

By channel board do you mean the little driver board with the row of pins along one edge, roughly the size of a pack of guitar strings? Are you saying that if you plug the B driver board into the A power module then A starts blowing fuses?

Or does module B blow fuses with either driver card?

First, go to the output terminal posts. Measure resistance between them. Are the B posts shorted together? When a channel blows, it often shorts the protective triac which is on the output terminals. If the red and black posts are shorted together, then you probably have a shorted SAC187 triac on the circuit board they are soldered to.

There are two rows of power semis on the module heat sink. Most are outputs, but the end one of each row is a driver transistor. Check each one for shorts. There are also a bunch of 10w ceramic resistors. Mostly low resistance like 0.33 or something. Check ALL of them for opens.

There is a diode on the heat sink with two wires running from it up to the driver board. Make sure it is not open. It is a dual diode, so the voltage drop across it will be twice what would be "normal."

Education is what you're left with after you have forgotten what you have learned.

Yes, I guess I am referring to the driver card. And the driver card from Channel B will blow fuses, regardless of which power module it is hooked up to. The driver card from Channel A will NOT blow fuses, regardless of which power module it is hooked up to.

As for the semis on the heat sink, I tested them all and found one open (it was an output, not a driver on the end), and replaced it. The problem persisted even after I replaced it.

I will check the diode, the ceramic resistors, and the triac, then post back.

Hi, sorry it took so long to get back -- had to take a break from the project.

I removed the protective Triac from Channel B. I hooked up the Channel B driver card back up in its original place -- any transistors that tested bad have already been removed and replaced with good ones from Channel A. (Channel A is not receiving power at the moment.)

I also tried swapping out the Driver Boards.

The fuse is still blowing, in the exact same fashion as before. What should I be looking for now?

The bias circuit is on the driver card, but that diode on the heat sink is part of it. Of course that particular diode doesn't travel with the driver card. SO if the good card will make that heat sink assembly work, then the heat sink must be OK, including its diode.

Let's review. As it stands you have one driver card that is bad and one that is OK. And we have determined that the triacs on both output terminal post boards are either OK or removed, right? And did we determine that both heat sink assemblies will work with the good card? or does one of those still have a problem too?

Look at the schematic. Two transistors on the card are called drvier transistors. They are really pre-drivers, since the drivers are on the heat sink, but that's OK. When one of these cards blows fuses, I check every darn semiconductor on it. The transistors AND the diodes. If any transistor was burnt out, then look for open resistors associateed with it.

Note there are pairs of transistors wired in parallel - Q3 and 5, then Q4 and 6. Those are difficult to test in the circuit because of all the parallel junctions.

When troubleshooting, disconnect the wires from the triac board. We don't need nor do we want a load until the fuse thing is cured. With no load, the output can swing to DC without blowing a fuse. No load, no current drawn, the amp sits ther with 50v on the output. But if it still blows fuses that way, we can assume both polarity outputs are being turned on at the same time. And that generally means the bias has let go.

From the base of Q9 to the base of Q10, Cr11,CR12 and the little resistor R30 maintain a voltage "space" between them. If one of those opens, or the pc traces do, then the two polarities are not longer held in control and both sides will go ON full. Fuses blow.

Education is what you're left with after you have forgotten what you have learned.